The present invention relates in general to a cleaning spray system which employs a solid carbon dioxide (snow) spray mixture stream, physicochemically modified to contain reactive inorganic gaseous species, which is directed at variable velocity, spray temperatures, and pressures onto substrate surfaces of components or articles that require cleaning and substrate treatment to allow for better bonding, gluing, markability, paintability, coatability or pottability. Various embodiments are incorporated herein which enhance the utility of the present invention.
The joining or bonding of substrates is a surface phenomenon, therefore surface preparation prior to bonding is critical for successful bonding. The sole purpose of surface preparation is to attain adherend surfaces receptive to the development of strong, durable bonded joints. It is desirable to have the basic adherend material (clean native substrate surface) exposed directly to the bonding agent (i.e., adhesive), coating agent (i.e., thin film), soldering agent (i.e., molten solder) or, in the case of acoustic: welding, a second clean and treated adherend. The absence of a second intervening layer such as an oxide film, particle, coating or release agent is often desirable. Conventional surface preparation processes for bonding typically involves two separate substrate surface treatments as follows: 1) surface cleaning to remove gross or trace surface contamination such as old coatings and paint and/or thin film hydrocarbons and particulates, and 2 surface modification to increase surface free energy (wetting) to promote contact between newly applied bonding agents and adherends. Achieving adequate adhesion to polymeric (organic), ceramic, glass and metallic (inorganic) adherends is a recurring and difficult problem throughout many industries. Many cleaning and modification processes have been developed and are discussed below.
Historically, various surface treatments have been used to improve the adhesion of coating or bonding agents to plastics. These include flame treatment, mechanical abrasion, solvent cleaning or swelling followed by wet chemical etching, or the application of specialized coatings in the form of chemical primers. Often what works for one specific application will not be effective for another, thus specific treatments need to be developed for each. For example, flame surface treatments present fire hazards and may damage heat-sensitive substrates. Solvent cleaning employing hazardous organic solvents such as acetone, toluene and methyl ethyl ketone (MEK) and acid or alkaline etching solutions present flammability, operator safety and/or ecological hazards, or may damage the substrate. Mechanical abrasion creates a particle aid residue clean-up issue and may damage critical surface topography. Moreover, the use of chemical primers requires specialized formulations for each type of polymer substrate.
Ceramics, pyroceramics and glasses are scrubbed with Ajax cleanser, or equivalent abrasive cleaner, rinsed with deionized or distilled water and dried at 120 to 150 F. Metallic substrates may be solvent cleaned using xylene, methyl ethyl ketone (MEK), or isopropyl alcohol (IPA) and air dried. Bare copper alloys are typically vapor degreased to remove gross soils, dipped in a nitric acid/ferric chloride solution to remove metallic oxides, rinsed with tap water to remove cleaning agents, spray rinsed with deionized water and finally air dried at 120 to 150 F. Alternatively, bare copper may be abrasively blasted with silica particles to remove metallic oxides, rinsed with deionized water to remove abrasives, and air dried at 120 to 150 F. Fluorinated polymers are wiped with acetone to remove gross surface contaminants, treated with sodium-napthalene solution, rinsed with acetone rinsed with deionized water to remove acetone residues and air dried at 100 F. Other common substrate treatment solutions include an FPL etch, which is a sulfuric acid dichromate pickling solution, and alkaline treatments such as sodium hydroxide-ferricyanide solutions. With some polymeric substrates, an extended strong organic solvent soak is necessary to produce a high energy surface layer which can be wetted by an adhesive or coating.
Consistent surface modification requires, in most cases, a fairly clean substrate—free of gross hydrocarbon contaminants and monolayers present on the uppermost surface layers. However, this requires performing a cleaning step independent of and prior to surface treatment. For example, solvent cleaning is acceptable for cleaning most substrates free of organic contaminants but has limited utility where a distinct change in the chemical nature of the substrate surface is required.
Where chemical treatments cannot be used due to part geometry, sensitivity or compatibility, and/or environmental risk, plasma etch techniques may be employed. However, a typical pretreatment prior to plasma etch is to remove oil, grease and other surface contaminants using an organic solvent such as 1,1,1 trichloroethane, toluene or MEK. Following this, the substrate is exposed to a gas plasma for 5 to 10 minutes at between 1 and 10 watts/cu.in. and under atmospheres of between 1 to 2 torr comprising oxygen, argon or water vapor and mixtures thereof.
Moreover, sand blasting, sand paper, abrasive pads or other mechanical abrasion techniques may be employed in place of chemical treatments. Similar to plasma etch treatments discussed above, the substrate must be degreased prior to mechanical abrasion, and following treatment, the residual abrasive agents must be removed from the surface. As such, multiple and separate steps are required to clean and prepare a substrate surface for bonding operations. Although, mechanical abrasion will remove hardened surface layers such as old polymeric coatings, paint, adhesives and metal oxides, it may not necessarily adequately treat the underlying exposed substrate surface. For example, activated plasma treatments discussed above using oxygen argon or water vapor plasma have demonstrated bond strengths three to four times that of abrasive surface preparation techniques. As such, both of these surface cleaning and treatment methods may have to be used in sequential order to properly clean and modify a surface in preparation for bonding.
For example, a typical substrate surface cleaning and modification application is the removal of old conformal coatings, for example parylene (an organic polymeric coating used to hermetically seal an electronic package), to enable the replacement of an electrical component mounted on an electronic substrate (i.e., BGA de-soldering, replacement and resoldering operation). In this common rework application, the parylene coating covering the BGA is selectively stripped using a chemical or abrasive agent. Following this, the BGA is de-soldered from the surface, the surface is cleaned and modified to promote wetting by solder and new conformal coating. A replacement component is re-soldered, and a new coating of parylene is applied and cured. This is exemplary of commercial production practices that require the iterative steps of stripping, cleaning, and modification.
More energetic, ecologically-friendly and worker-safe alternatives have been developed. These include, high energy density treatments such as ultraviolet (UV) radiation (with/without ozone) and atmospheric plasma have gained greater acceptance on a larger scale for substrate surface modification. They provide a medium rich in reactive species, such as energetic photons, electrons, free radicals, and ions, which, in turn, interact with the polymer or metallic substrate surface, changing its surface chemistry and/or morphology. However, these newer processes require that the substrate be free of gross contamination, wherein some type of conventional surface cleaning process is still required prior to use of these high energy surface modification processes. One example of a conventional surface modification system is the PT-2000 Plasma Treatment System from Tri-Star Technologies, El Segundo, Calif. The device uses a supply of nitrogen, argon, oxygen and/or other gases, singularly or in combination, in combination with an electrical corona generator and suitable corona forming nozzles to create a gas jet plasma or atmospheric plasma stream. The atmospheric plasma stream is then directed at a substrate thereby modifying its surface—creating high surface free energy and implanting functional groups into the surface layers, depending upon the plasma gas phase chemistry used. Different plasma gases and gas mixtures provide different surface properties. As such, the desired surface treatment can often be optimized for a particular surface, and bonding process. Recommended pre-cleaning of the surface is either a solvent wipe or aqueous wash and dry.
An example of an energetic surface cleaning and modification technique is given in U.S. Pat. No. 5,054,421, Ito, et al. In this process, a substrate is cleaned using a gas jet which is simultaneously irradiated with an electron beam. The gas molecule are excited through the creation of a glow discharge (plasma), following which the reactive gas mixture is contacted with the substrate. A voltage of up to 2000 volts may be applied between two electrodes to produce the glow discharge. The process is performed under a depressurized condition, 0.1 to 10 mm Hg vacuum, and elevated temperatures using various inorganic and organic gases and mixtures, for example silane and oxygen. An electric field imparts momentum and accelerates the reactive gas mixture, under vacuum, at the substrate surface. The '421 process teaches both cleaning and coating the substrate with thin films (i.e., silicon) using the invention. The process as taught is performed under low pressure conditions to impart ionization to the cleaning gases (i.e., oxygen—O.sub.2) or thin film forming gas (i.e., silane—SiH.sub.4). The main drawbacks with this invention as it relates to surface cleaning and treatment is that it must be performed within a vacuum environment, necessitating the use of expensive vacuum chambers, pumps and environmental controls. As with most conventional vacuum plasma cleaning processes, the substrate surface must be significantly absent of gross hydrocarbon contaminants to prevent the contamination of the vacuum environment and to allow the reactive gases to contact the underlying substrate surface.
From the above, it can be seen that a method and apparatus for cleaning and treating various medical, electronic and mechanical substrates that offers enhanced cleaning and surface modification under standard temperature and pressure conditions, and is safe, easy, and reliable and can be easily integrated with automation and control systems for inline production applications is often desired. Moreover, it is desirable to have a process, method and apparatus that can be integrated with conventional bonding agents and/or processes to allow for in-situ surface cleaning and modification with bonding, coating, painting and curing. As such, there is a present need to provide processes, methods and apparatus for simultaneously cleaning and modifying a substrate surface in preparation for bonding, gluing, marking, painting, coating, potting and curing.